What is "Cross Tripping" in speedtronic Mark iv controlled gas turbine?

 

I have heard of TCEA cross trip which directly relates to L4_XTP. Refer your control sequence for details with this description

 

MOB,

The questioner asked about cross-tripping related to Speedtronic Mark IV turbine control systems. Mark IV's don't have TCEA cards or use the L4_XTP logic signal name, and don't have control sequence (programs) like Mark V's.

The Mark IV used a logic signal called L4X1 (I think) to establish the P28VR.4 bus to allow the "protective" solenoid relays (like the electromechanical relays for 20CB-1, 20FG-1, 20FL-1, etc.) to be energized. If L4 dropped out, the P28VR.4 bus was de-energized and the "protective" solenoid output relays dropped out.

L4X1 was "driven" by L4; when L4 is "1" then L4X1 is "1". When L4 is "0" then L4X1 is "0". The concept was that a software trip would drop out the hardware relays to trip the turbine, as I recall.

You can "see" the P28VR.4 bus on the 04x (usually 04G for TMR Mark V turbine control panels) sheet, and also on the 20x sheets (usually 20A and 20B). To get some of the solenoid output relays (partial list above) to pick up, it was necessary to have the driving logic picked up and also L4X1 in order to get the P28VR.4 bus power for the electromechanical relay coils.

It's sort of similar in the Mark V, but GE inverted the logic (L4_XTP goes to a logic "0" when L4 is a logic "1", and to a logic "1" when L4 is a logic "0"). L4_XTP must be a logic "0" as part of the logic to get the ETRs (Emergency Trip Relays) to pick up in the Mark V <P> core on the TCTx (TCTG, TCTS, TCTL) card via the TCEA cards.

We need to be specific in our responses and provide detail. That's why people come here and keep coming back and tell their colleagues about this site.

 

Thanks CSA as always for the detailed explanation. Actually I just overlooked that it was Mark-IV. Can you please provide more insight with regard to the L4_XTP i.e with regard to the ETR & PTR's. Does this issue relate to <P> and not <Q> core. Pl. explain

 

The idea behind the PTR/ETR scheme is to have a means of tripping the unit from both the sequencing (the PTRs) and the <P> core (the ETRs).

The PTRs (Primary Trip Relays) are driven by sequencing in <Q>. An example would be the PTR for the Gas Fuel Stop Valve Trip Solenoid, 20FG-1. The CDB signal name for that is typically L20FG1X, which drives the PTRs for 20FG-1. The PTRs provide the negative 125 VDC for the trip solenoid.

The ETRs (Emergency Trip Relays) are driven by firmware in <P>. The Emergency Electrical Overspeed Trip, and another speed-related trip signal, the rate of acceleration/deceleration (which is not adjustable), can cause the ETRs to be de-energized when the Master Protective logic, L4 is energized.

In later Mark V sequencing, the cross-trip was "added" to the L4 rung as a parallel, inverted coil, to make it "easier" to understand. By having the PTRs "connected" to the state of L4 with L4_XTP, it also ensures that a trip detected by the sequencing will also de-energize the ETRs.

There's no "protective" equivalent of L20FG1X; the cross-trip serves that function.

In writing this, it seems that a way to energize the ETRs when L4 was not energized (such as during maintenance activities like LVDT calibration of the SRV) was necessary. And this cross-trip scheme kind of lent itself to that task.

For example, say it's necessary to get 20FG-1 to be energized to establish Gas Fuel Trip Oil pressure when the unit is on cooldown. Both the PTRs and ETRs must be energized. Simply forcing L20FG1X (which only energizes the PTRs for 20FG-1) by itself will not energize 20FG-1.

One can energize the ETRs by forcing L4_XTP to a logic "0". If there are no trips detected by the <P> core firmware, and the E-stop string is not broken, then the ETRs will be energized to allow the 125 VDC to pass to the trip solenoids. This, in conjunction with forcing L20FG1X to a logic "1" will energize 20FG-1.

Does this help? The need to be able to trip the unit from the <P> core independently of the sequencing is pretty easy to understand. For some reason, it became necessary to ensure that the ETRs would also be de-energized if the sequencing detected a trip condition, and the cross-trip serves that function. It also was "useful" in allowing a method to energize the ETRs during maintenance activities, though the whole inversion thing just never made sense. (To me, anyway.)